Polycarbonates are generally used as materials for the injection molding or the injection-pressure molding of optical data carriers on account of their particular combination of properties such as transparency, heat resistance and dimensional stability. To improve processability, which generally takes place at temperatures in the range of 300° C. to 400° C., additives such as mold release agents and stabilisers are added to the polycarbonate.
Aliphatic fatty acid esters of polyhydric alcohols, for example esters of glycerine with long-chain fatty acids and of pentaerythritol with long-chain fatty acids are preferably used as mold release agents. These esters may also be partially esterified in which case they possess free OH groups which are obviously advantageous for the mold release action, but these esters impair the heat stability of the mixture so it is necessary to add stabilisers. However, this stabiliser must be very effective, i.e. must be effective in very low concentrations as there is otherwise the risk of coating formation in the mold. In this case, the molds and dies often have to be cleaned during processing, and this is detrimental and leads to sheets of inferior quality. Completely esterified mold release agents, on the other hand, have higher stability, but the mold release action of these esters at low concentrations is much weaker than that of esters with free OH groups. Therefore, greater quantities of completely esterified mold release agents generally have to be used, and this increases the risk of coating formation but, on the other hand, the amount of stabiliser added may be somewhat smaller.
A plurality of mixtures have already been proposed in the literature, such as:
For stabilising mixtures containing mold release agents with OH groups, EP-A 205 192 teaches the use of mixtures of trimethyl phosphate and/or triethyl phosphate combined with phosphites. Accordingly, the stabilising effect of the phosphoric acid esters or the phosphites alone is inadequate and these phosphoric acid esters are ecologically undesirable.
JP-A 62184639 describes optical data memories produced from polycarbonate stabilised with phosphites. Readily volatile phosphites such as trimethyl phosphite, which are ecologically undesirable and critical for coating formation are preferred.
JP-A 2000080261 describes polycarbonate mixtures with mold release agents, tris(di-tert-butyl-phenyl)phosphites and phosphoric acid. Accordingly, the stabilising effect of the phosphite is inadequate, so phosphoric acid is used as further stabiliser, but it can cause corrosion of the memory layers on the optical data carriers under conditions of high atmospheric humidity and elevated temperature.
It was accordingly the object to develop thermoplastic polymer mixtures containing polycarbonate, mold release agents and heat stabiliser, which are optimised for the production of optical data carriers and other shaped articles, in other words, for example, have good mold release action in smaller quantities. In particular, the new disc format with higher storage capacity and optionally thinner discs such as digital versatile discs (DVDs) demands higher heat stability than CDs. Damage to material during processing to shaped articles and coating formation in the mold become more critical.
With the mixtures according to the invention, this object is surprisingly achieved by improved quality of the data memories and improved processibility of the material during injection molding or injection pressure molding.